The line of neatly-shorn young men in dark blue suits began forming at 2:30 a.m. today at the front door of the Tokio Marine and Fire Insurance Co. Ltd, in Tokyo's business center.

Six hours later, it had grown to 374 men and stretched out across the company's outdoor patio and along two adjacent sidewalks.

Promptly at 8:30 a.m., the doors were pulled open by a guard, the men in blue suits stiffened and filed insider, and Japan's annual college graduates' job hunt was under way.

Under a voluntary system established by gentlemen's agreement five years ago, all students scheduled to graduate the following spring are supposed to start looking for employment on the same autumn day. Some cheat and sneak in early and a few corporations also break the rule, but most of the class of 1978's 275,000 job-seekers met personnel managers for the first time this morning.

Even in good time, when jobs are plentiful, it is a time of tension and anxiety. It is doubly so this year when fewer graduates than usual can find desirable openings. Toshiko Shimmhara, 21, showed up outside Tokio Marine at 3:10 a.m. because he wants a stable company that "takes care of its employes." Takashi Yoshino, a Tokyo College of Economics student, also came early, "This is the worst year for students to find jobs," he said.

A Japanese man usually spends his career working for the same enterprise, so the company visitations take on enormous importance.

Preparations begin weeks in advance at college placement centers. Toshio Chiba, placement director at Sophia University, takes a 5:30 a.m. train to arrive at his office early so that he can advise more students. Last week, he found students lining up outside his door at 4 a.m. At Meiji University, hundreds of worried-looking students grouped around bulletin boards where corporations had posted lists of jobs available.

Job-seekers adopt the vocabulary of war to describe their searches. One said he hopes to "attack" the companies in the automobile industry. At Meiji last night, 3,000 students gathered for what was formally described as a ceremony "to congratulate students just before leaving for their battlefields." After a placement official's pep talk, cheerleaders led the group in singing the university song.

Some students take extraordinary measures to attract corporate attention. Terumi Takano, a Meiji student, mentioned a classmate who broke the deadline by going to a medical company and refusing to leave its lobby until personnel officers agreed to meet with him.

Takano himself jumped the gun by visiting two phonograph record companies recently. "Unfortunately, it seems they didn't accept me," he said. "I have to apply through the official route."

Last spring, one young woman bounded onto the table of her university dining hall and asked for help from any classmates who had connections at a television company where she had hoped to work. Someone volunteered a contact and she got the job.

Members of Parliament find their reception rooms crowded with job-seekers whose parents are influential constituents. They obligingly write letters to company executives urging interviews for the favored students.

Isao Nishi, placement director at Meiji University, says the politicians' letters do not do much good, but his advice does not prevent students from trying.

Some students break the rule by approaching business executives under the guise of doing "corporate research." Others seek private interviews through relatives who take them to the homes of top industry officials.

Such tactics are common this year because the economic slump has sharply reduced hiring in many industries. A Labor Ministry survey of corporate hiring plans discovered that only 57 per cent of major companies plan to employ any new university granduates next spring, compared to last year's 83 per cent.

Japan's company visitation system began in 1974 as an attempt to reduce some of the fierce competitiveness in the corporate hiring field. Educaton officials found that anxieties about careers were ruining the students' effectiveness during their third year of college - they were too busy interviewing at personnel offices to get much studying done.

"It's a good system," said Tomohide Madenokoji, a public relations official for Tokio Marine and Fire Insurance.

"If everyone kept the rule, we would have a fair competition and both sides (students and employes) would conserve energy. If we did not have such a system, the competition would begin in summer. It would ruin the students' lives."